1. Field of the Invention
The present invention relates generally to optical waveguide lenses used for collimating or focusing a light beam traversing free space in an optical component, and particularly to a spherical lens formed on the distal end of an optical fiber.
2. Technical Background
Advances in optical communications have generated significant need for optical components which involve light being transmitted to or from an optical fiber through free-space to interact with or pass through one or more optical device. A wide variety of passive and active optical devices exist, some simple examples including thin-film filters or birefringent elements, and some of the more complex being large scale three-dimensional switch fabrics. Other uses for free-space optical components include injecting light from a laser diode into an optical fiber, transmitting light through free-space from one optical fiber to another (such as an amplifier fiber), or projecting light from an optical fiber to a detector.
In optical components utilizing free-space transmission of light (sometimes called micro-optic components), the light beam is often either expanded and collimated into approximately parallel rays from the exposed end of an optical fiber, or conversely focused from an expanded beam into a narrower beam capable of being injected into the end of the optical fiber at a desired angle of incidence. While other functions may be performed on the light beam exiting or entering an optical fiber, collimating and focusing are the functions which are most commonly encountered in micro-optic components.
In order to accomplish the collimating or focusing functions within the specifications required for optical communications, cylindrically-shaped gradient-index (GRIN) lenses employing a graduated radial refractive index profile have become the most prevalent conventional alternative. However, commercially-available GRIN lenses are expensive, difficult to manufacture, and present certain disadvantages in assembling, aligning, and tuning the optical components.
Several other approaches to fabricating collimating or focusing lenses for optical components are known. Axial GRIN lenses, molded polymer and glass lenses having spherical and aspherical lens surfaces, composite or complex lens elements, optical fibers having integral lenses formed by processes such as thermal expansion or diffusion, and ball lenses are among the many alternatives.
One approach to producing a larger-diameter collimating or focusing lens attached to the end of an optical fiber involves fusing a cylindrical glass rod onto the end of the optical fiber, and heating the rod above its softening point so that surface tension forms the viscous melted glass into a spherical lens. The resulting lens will usually have a cylindrical section of equal diameter with the optical fiber, and a spherical section of larger diameter. A light beam exiting the end of the optical fiber will expand in the cylindrical portion of the lens at a rate dependent on the refractive index of the material, and continue expanding through the spherical portion until refracted at the interface between the spherical lens surface and the surrounding air. The expanded or collimated beam is approximated as parallel rays at an operating distance from the lens surface, but in fact has a region of progressively decreasing beam waist as the initial displacement from the lens surface increases. Spherical fiber lenses fabricated according to this technique have provided beam expansion by Gaussian diffusion from the 10 μm core of standard 125 μm diameter single-mode optical fiber to an expanded beam diameter of approximately 80 μm at the exit face of a lens having a 200 μm radius of curvature, with the beam converging to an approximately 30 μm beam diameter at the beam waist located 0.65 mm from the lens surface. For spherical fiber lenses having up to about 350 μm radii of curvature, beam diameters of up to about 120 μm could be achieved at a beam waist located approximately 4.5 mm from the lens surface, implying a 9 mm separation between a pair of collimating lenses having an optical device disposed in between within an optical component.